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Topic: Bullet inlet tract extension (Read 4973 times)

I just completed and tested my latest mod, which is an inlet tract extension on my carburetor.It essentially performs a similar function as a velocity stack, but perhaps a little less perfectly. However, it allows me to use my existing K&N air filter on the end, because it does not have a flared bellmouth on it. So, I get the inlet tract length increase that I wanted, and still get to use my air filter.I felt that I'd get enough benefit from the constant-diameter tube to suit my needs, and wanted to keep the filter on the end, so that was the basis for the decision making process.

There are a couple main targets for the inlet tract increase.

One is very simply to permit a larger amount of air volume on the "engine side" of the filter barrier. The stock position of the K&N filter is right on the carb mouth, and the only volume inside the filter barrier is basically the volume of the carb throat and the inlet port. This does not amount to even 500cc, which would be the minimum volume needed for "one big gulp" of air when opening the throttle quickly. Having more air volume inside the filter barrier can provide better throttle-response because it doesn't have to draw air thru the filter for the first "gulp". It's common practice to provide greater air volume than the engine displacement inside the filter barrier for this purpose.So, it accomplshed that goal at the very least. The "pod type" filters such as the K&N are less than ideal in their normal mounting position on the carb, because of this factor.

Another target is to achieve a "ramcharging" effect, by the inertia of this moving air inside the inlet tract. Once this "cylinder of air" in the inlet tract gets moving, it continues to move from inertia, even when the piston stops drawing air in. This assists the filling of the engine cylinder by using all the available time that the inlet valve is open, to get the most air into the engine.

And lastly, it is for "resonance wave tuning", which uses the reflection of sound waves that reflect up and down the intake tract during running. By tuning the wave behavior in the inlet tract by using certain lengths, the waves can be used to assist the flow of the air into the cylinder, in a very simlar way that exhaust length tuning helps to extract the exhaust out of the engine. Just the other side of the same coin, so to speak. The longer tract can bring the reflection frequency down lower in the rpm band, where it can be better utilized by the Bullet's slow-revving engine design. The short inlet tract in the stock engine is not optimized in any way but for fitting on the motorcycle. It's too short to provide any real wave-tuning benefits in the low rpms that we can access. Short inlet tracts serve higher rpms better, and we can't even run our engines at rpms that would be good for an inlet tract length that is as short as the stock Bullet has.Lengthening this inlet tract can work out as beneficial for us in the low rpms that our engine can use.

So, for a very cheap cost, some benefits can be achieved for the actual running improvements of the bike. It increases torque and hp at certain rpm points, and we strive to time this activity to coincide with the rpm of our engine's torque peak.Actually, the length that I am using is still a little shorter than optimal, because I ran out of room to put the extension, without it hanging out too far off the side of the bike. So, I selected a 6" length, which would be good for rpms slightly higher than the torque peak, but not too much higher, and still useful, while fitting into the available space and visual appearance parameters.

For those interested, I used the stock rubber bellows that usually connects the carb throat to the stock air filter housing, to connect the stack to the carb. I used a rubber o-ring as the "clamp" to hold it on the carb.The tube is nothing more than a 1.75" I.D. tube, cut to 6" length. The K&N just fits on the end, because the tube is the same diameter as the carb throat body, and it just clamps right on.I used a cable tie to secure the "flying end" to the frame, so that it didn't sag and fall off, or over-stress the intake rubber manifold part from the added weight and distance. Gives the whole thing more support and stops it from falling off due to vibrations. You could use a mounting clamp or bracket for this purpose too. I just used a cable tie, and it seems to work ok.

Now, I realize that this may not be a pretty as Chumma7's stack, nor probably quite as ideal in function. Chumma's stack is really nice. But I wanted to have the K&N on the end, so I adjusted the concept to fit my need.

Here's a photo of it on my bike.

« Last Edit: December 07, 2008, 05:50:25 PM by ace.cafe »

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geoffbaker

general thought... on my diesel and the scoop, I can build in some sort of extension (though it might have to turn some corners). What difference do you think there would be between using flexible tubing (with its spiraling shape) vs a smooth bore tubing of some kind?

Interesting write-up, Ace. It accords with my abortive attempt to install that kind of filter close up on the carb. Even though I had gone up a couple of jet sizes, I found I was getting dreadful just-off-idle to 1/4 throttle response - the mother of all flat spots - so I went back to the stock air filter, reluctantly because I wanted the tool box back for putting tools in!

I might fashion a similar tract extension and see if there is an improvement.

general thought... on my diesel and the scoop, I can build in some sort of extension (though it might have to turn some corners). What difference do you think there would be between using flexible tubing (with its spiraling shape) vs a smooth bore tubing of some kind?

General rules would be to provide the widest sweeping bends possible in the application, and use smooth-bore tubing.Any type of tubing that won't collapse from the intake vacuum, and won't be adversely affected by the fuel will be fine. It doesn't have to be metal.The spiral type tubing which looks like vacuum-cleaner hose, has the effect of reducing the effective working diameter of the hose, because the spiral ribbing inside creates eddys and vortices at the walls of the tube, and forcing the main airflow to concentrate more in the center. This causes the spiral ribbed tube to effectively act as a smaller I.D. tube, dynamically.

If you want to use a scoop, make a wide sweeping "U" shaped inlet tube that is long, and ends up pointing it's mouth forward. Put your K&N or whatever filter pod you have on there, with a coffee can around it, also pointing forward, with the air filter inside and the hose thru a hole in the middle of the bottom of the coffee can, and you have your "scoop". This creates a high-pressure zone inside the coffee can when you are riding, and gives you the "ram air" effect, and you get the extended inlet tract benefits along with it, and you can use your filter with it too.Mount the coffee can to your front frame downtube with a bracket, for cold intake air, paint it black, and you're on your way for a few bucks.

Interesting write-up, Ace. It accords with my abortive attempt to install that kind of filter close up on the carb. Even though I had gone up a couple of jet sizes, I found I was getting dreadful just-off-idle to 1/4 throttle response - the mother of all flat spots - so I went back to the stock air filter, reluctantly because I wanted the tool box back for putting tools in!

I might fashion a similar tract extension and see if there is an improvement.

Chas

Chasfield,Interesting!Because I too, had a flat spot like that after installing the K&N on the carburetor.This mod eliminated that problem, and it was the first thing that I monitored to see what effects it would have on that. Problem solved by this, in my case.

I don't really think that the inertia/momentum of a 6" column of air would offset the inductive effect of extending the filter into more into the airstream on your setup. I am thinking of inline inductors used to dewater spaces on ships or such. I think even with your leg out in front of the filter this would be an issue.

Also I I tend to think the breathing issue is not really helped all that much by increasing the volume on the engine side of the filter when we are talking about such a low volume. Your will still have to replace the displaced air through the filter medium and taking into account the actual permeable surface area of the KN or some such filter I believe this would also be a negligible improvement. I was under the impression that most of the stock airflow restrictions were created by the box/multiple boxes employed in the system.

I think it would be better to get some of flexible header pipe (available at Napa / Autozone and such) and bend it around 180 degrees into the airstream with the addition of a scoop to create a true ram air effect. Of course this adds the issue of ingesting large volumes of water when it rains etc.

I don't really think that the inertia/momentum of a 6" column of air would offset the inductive effect of extending the filter into more into the airstream on your setup. I am thinking of inline inductors used to dewater spaces on ships or such. I think even with your leg out in front of the filter this would be an issue.

Also I I tend to think the breathing issue is not really helped all that much by increasing the volume on the engine side of the filter when we are talking about such a low volume. Your will still have to replace the displaced air through the filter medium and taking into account the actual permeable surface area of the KN or some such filter I believe this would also be a negligible improvement. I was under the impression that most of the stock airflow restrictions were created by the box/multiple boxes employed in the system.

I think it would be better to get some of flexible header pipe (available at Napa / Autozone and such) and bend it around 180 degrees into the airstream with the addition of a scoop to create a true ram air effect. Of course this adds the issue of ingesting large volumes of water when it rains etc.

My 2 pennies...

Interesting points.

First, it's hard to see from the perspective, but it is located behind my leg as you surmised in your comments. As such it is in an area of turbulent air anyway, so as to minimize any "drawing-out" induction effects from the airstream. I'm not totally discounting that possibility though, and I'm not seeking "ram air" in the sense of pressurized intake from the external airstream, in any case. I'm simply seeking the known effects of added intake tract length, which is standard engineering practice.I offer "Exhibit A"

Also, it is standard engineering practice to have airbox volume inside the filter barrier which exceeds engine displacement, often by a factor of 2, for the very purpose I described. You'll notice on virtually any factory-built vehicle, a volume greater than displacement on the engine side of the filter. Even on sportbikes, where space is at a premium.

So, while I have no dyno, or other way to fully determine the effects, I feel comfortable in relying on proven engineering practice in the automotive/motorsports field for my modifications.

« Last Edit: December 07, 2008, 06:45:41 PM by ace.cafe »

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geoffbaker

General rules would be to provide the widest sweeping bends possible in the application, and use smooth-bore tubing.Any type of tubing that won't collapse from the intake vacuum, and won't be adversely affected by the fuel will be fine. It doesn't have to be metal.The spiral type tubing which looks like vacuum-cleaner hose, has the effect of reducing the effective working diameter of the hose, because the spiral ribbing inside creates eddys and vortices at the walls of the tube, and forcing the main airflow to concentrate more in the center. This causes the spiral ribbed tube to effectively act as a smaller I.D. tube, dynamically.

If you want to use a scoop, make a wide sweeping "U" shaped inlet tube that is long, and ends up pointing it's mouth forward. Put your K&N or whatever filter pod you have on there, with a coffee can around it, also pointing forward, with the air filter inside and the hose thru a hole in the middle of the bottom of the coffee can, and you have your "scoop". This creates a high-pressure zone inside the coffee can when you are riding, and gives you the "ram air" effect, and you get the extended inlet tract benefits along with it, and you can use your filter with it too.Mount the coffee can to your front frame downtube with a bracket, for cold intake air, paint it black, and you're on your way for a few bucks.

So if I use a larger diameter spiral hose I can offset the spiral wall issues...I'm just thinking that with the space restrictions it may be impossible (cost effectively that is) to build anything without hose that turns easily in the tightest corners. Rubber tubing would require many sections and turns and clamps to achieve the same thing.

COFFEE CAN? On my Enfield??I think not. Perhaps a fat piece of PVC pipe would be more classy

General rules would be to provide the widest sweeping bends possible in the application, and use smooth-bore tubing.Any type of tubing that won't collapse from the intake vacuum, and won't be adversely affected by the fuel will be fine. It doesn't have to be metal.The spiral type tubing which looks like vacuum-cleaner hose, has the effect of reducing the effective working diameter of the hose, because the spiral ribbing inside creates eddys and vortices at the walls of the tube, and forcing the main airflow to concentrate more in the center. This causes the spiral ribbed tube to effectively act as a smaller I.D. tube, dynamically.

If you want to use a scoop, make a wide sweeping "U" shaped inlet tube that is long, and ends up pointing it's mouth forward. Put your K&N or whatever filter pod you have on there, with a coffee can around it, also pointing forward, with the air filter inside and the hose thru a hole in the middle of the bottom of the coffee can, and you have your "scoop". This creates a high-pressure zone inside the coffee can when you are riding, and gives you the "ram air" effect, and you get the extended inlet tract benefits along with it, and you can use your filter with it too.Mount the coffee can to your front frame downtube with a bracket, for cold intake air, paint it black, and you're on your way for a few bucks.

So if I use a larger diameter spiral hose I can offset the spiral wall issues...I'm just thinking that with the space restrictions it may be impossible (cost effectively that is) to build anything without hose that turns easily in the tightest corners. Rubber tubing would require many sections and turns and clamps to achieve the same thing.

COFFEE CAN? On my Enfield??I think not. Perhaps a fat piece of PVC pipe would be more classy

Theoretically a larger spiral hose would flow sufficiently, but I really don't know what the total diffraction effects of the spiral on the sound waves would do.I have doubts.

Check Lowe's Hardware. I know they have very flexible hose of different types, and large diameters, that they will custom cut to lengths you desire, Not expensive.

Ace I tried the same mod last year, used sewer pipe pvc, and heated the end to form fit the carb base. It worked well, I started at 12" and finally ended up with 3 3/4 ",cutting off 1/2 "at a time. My K&N was a much larger filter, and I was able to flare a bell on the end kinda like the bell on a stack. I will try moving the carb back from the head next,spacing the manifold.I am trying for torque, not hi rpm power

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99 Enfield Bullet 535a man isn't drunk,if he can lie on the floor without hanging on

Ace I tried the same mod last year, used sewer pipe pvc, and heated the end to form fit the carb base. It worked well, I started at 12" and finally ended up with 3 3/4 ",cutting off 1/2 "at a time. My K&N was a much larger filter, and I was able to flare a bell on the end kinda like the bell on a stack. I will try moving the carb back from the head next,spacing the manifold.I am trying for torque, not hi rpm power

Dogbone,I like the idea of moving the carb out, but the damn tank is in the way of the top of the carb where the cable comes in.If you find a way around that, please post it.

Maybe you can find some large diamter conduit pipe for electrical purposes, locally.If not, Ebay usually has almost anything.Or McMaster-Carr has practically anything too.

Related to right-angle bends, that is not good for sonic waveguide purposes..They don't like right angle bends. A "sweeping turn" is better, and straight with no bends is best.But, if you have no other option, it might be better than nothing. In any case, it can get you the ram air you were wanting, even if the sonic activity gets thwarted with sharp bends.

geoffbaker

Maybe you can find some large diamter conduit pipe for electrical purposes, locally.If not, Ebay usually has almost anything.Or McMaster-Carr has practically anything too.

Related to right-angle bends, that is not good for sonic waveguide purposes..They don't like right angle bends. A "sweeping turn" is better, and straight with no bends is best.But, if you have no other option, it might be better than nothing. In any case, it can get you the ram air you were wanting, even if the sonic activity gets thwarted with sharp bends.

I need to build an intake pipe to mount to the squarish shaped engine air intake. That iwll have to turn 90 degrees backward, and the turn should occur in less than 3 inches. I'll make a flange to fit the intake, and weld the pipe to it. After that, I have already built a scoop box which I think will work, I just need to take some pipe and mount a filter in the box. The pipe will enter at the front of the box and turn down so the filter will mount beneath it inside the box.Once this is done I just need to tie the system together with some rubber hose and a carburetor and bob's your uncle. I'll post pictures when complete.